1. Field of the Invention
The present invention is directed in general to field of information processing. In one aspect, the present invention relates to a system and method for codeword retransmission.
2. Description of the Related Art
Wireless communication systems transmit and receive signals within a designated electromagnetic frequency spectrum, but the capacity of the electromagnetic frequency spectrum is limited. As the demand for wireless communication systems continues to expand, there are increasing challenges to improve spectrum usage efficiency. To improve the communication capacity of the systems while reducing the sensitivity of the systems to noise and interference and limiting the power of the transmissions, a number of wireless communication techniques have been proposed and/or adopted in several current emerging standards which use techniques, such as Multiple Input Multiple Output (MIMO), which is a transmission method involving multiple transmit antennas and multiple receive antennas.
An example of such a wireless system is the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) platform system 1 depicted in FIG. 1, which uses Orthogonal Frequency Division Multiplexing (OFDM) and multiple-input multiple-output (MIMO) antenna technology to implement the Evolved Universal Terrestrial Radio Access (E-UTRA) air interface. Generally speaking, the E-UTRA interface provides new functionalities or features for data channels (such as fast link adaptation, hybrid automatic repeat request, and simultaneous codeword transmission) that rely on rapid adaptation to changing radio conditions. As depicted, the infrastructure includes one or more transceiver devices 2, 4, 6, 8 which each include transmit and receive circuitry that is used to communicate wirelessly with any mobile end user(s) 10-15 located in each transceiver device's respective cell region. Thus, transceiver device 2 includes a cell region 3 having one or more sectors in which one or more mobile end users 13, 14 are located. Similarly, transceiver device 4 includes a cell region 5 having one or more sectors in which one or more mobile end users 15 are located, transceiver device 6 includes a cell region 7 having one or more sectors in which one or more mobile end users 10, 11 are located, and transceiver device 8 includes a cell region 9 having one or more sectors in which one or more mobile end users 12 are located. In the LTE architecture, the transceiver devices 2, 4, 6, 8 may be implemented with base transceiver stations (referred to as enhanced Node-B or eNB devices) which in turn are coupled to Radio Network Controllers or access gateway (AGW) devices 22, 24 which make up the UMTS radio access network (collectively referred to as the UMTS Terrestrial Radio Access Network (UTRAN)). Through the access gateway devices 22, 24, the eNBs 2, 4, 6, 8 are coupled to an EPC 26 (Evolved Packet Core) and switching center 28 of some form and from there to other public switched networks, e.g., public switched telephone network, Internet, or other packet and possibly circuit switched networks. As will be appreciated, each transceiver device (e.g., eNB 2) in the wireless communication system 1 includes a transmit/receive antenna array and communicates with receiver device (e.g., user equipment 15) having a receive antenna array, where each antenna array includes one or more antennas. The wireless communication system 1 may be any type of wireless communication system, including but not limited to a MIMO system, SDMA system, CDMA system, SC-FDMA system, OFDMA system, OFDM system, etc. Of course, the receiver/subscriber stations (e.g., UE 15) can also transmit signals which are received by the transmitter/base station (e.g., eNB 2). The signals communicated between transmitter/base station(s) and receiver/subscriber station(s) can include voice, data, electronic mail, video, and other data, voice, and video signals.
To provide high rate packet data services over the E-UTRA air interface, each transmitter/base station (e.g., eNB 2) may include a scheduler or resource scheduler functionality for allocating resource blocks to users which are currently connected to the access network. The resource scheduling data and control signaling, along with other control information (e.g., power level, channelization codes, etc.), must be conveyed to each receiver device (e.g., UE 15) over a downlink channel so that the receiver device will know how to transmit data on the uplink. While the downlink signaling may be done in any desired way, the E-UTRA air interface at 3GPP TS 36.213 (V8.3.0) specifies physical downlink shared channel related procedures for transmitting downlink data and control information from the transmitter/base station (e.g., eNB 2) to the receiver/subscriber stations (e.g., UE 15). In particular, the transmission mode and downlink control information (DCI) is defined so that each receiver/subscriber station (e.g., UE 15) receives a Physical Downlink Control Channel (PDCCH) and a Physical Downlink Shared Channel (PDSCH) data transmission associated with predetermined DCI format signaled by the associated PDCCH. Thus, a receiver/subscriber station (e.g., UE 15) may be semi-statically configured via higher layer signaling to receive PDSCH data transmissions in a particular transmission mode associated with a reference DCI format signaled by a PDCCH in its UE-specific search spaces based on the following table:
TABLE 1Reference DCI Format(s)Supported By Each Transmission ModeTransmission ModeReference DCI Format1. Single-antenna port; port 01, 1A2. Transmit diversity1, 1A3. Open-loop spatial multiplexing24. Closed-loop spatial multiplexing25. Multi-user MIMO1D6. Closed-loop Rank = 1 precoding1B7. Single-antenna port; port 51, 1A
In accordance with Table 1, the transmitter/base station (e.g., eNB 2) can transmit a PDCCH to signal the receiver/subscriber stations (e.g., UE 15) that PDSCH data transmissions associated with DCI format 1A will be transmitted using a transmit diversity mechanism which supports sending a single transport block or codeword (CW) in a subframe. But if the channel conditions change, the transmitter/base station can dynamically switch from one format (e.g., DCI format 1) to another (e.g., DCI format 2), or vice versa. For example, if radio conditions improve, the transmitter/base station (e.g., eNB 2) can transmit a PDCCH to signal the receiver/subscriber stations (e.g., UE 15) that PDSCH data transmissions associated with DCI format 2 will be transmitted using spatial multiplexing which supports the transmission of two transmission blocks or codewords (CW) in one sub frame of a MIMO transmission.
One drawback associated with dynamically switching between transmission formats is the difficulty of identifying a transport block/codeword that is being re-transmitted in accordance with an error control protocol, particularly when only a single transport block/codeword can be re-transmitted to due changed channel conditions. For example, the current LTE standard provides a hybrid ARQ (HARQ) process in which retransmissions of the same transport block/codeword can be requested by the receiver/subscriber station (e.g., UE 15) by reporting a Negative Acknowledgement (NACK) in case a transport block/codeword is received in error in the first transmission. If the receiver/subscriber station reports NACK on a single codeword transmission that is transmitted using DCI format 1A, then it is a simple matter of retransmitting the codeword again using the same DCI format. But for the MIMO case in which two transmitted transport blocks/codewords are received in error and the transmitter/base station/eNB switches from DCI format 2 to DCI format 1A prior to retransmission, then only a single transport block/codeword can be transmitted at a time using DCI format 1A. Unfortunately, there is no mechanism provided in the DCI format 1A for identifying which of the two transport blocks/codewords is being transmitted. For example, the DCI format 1A has only a three-bit HARQ field, so it is not possible to use the HARQ field to identify both the full range of HARQ process ID values and the codeword number. While there have been proposals to signal which of the two transport blocks are being retransmitted by providing a mechanism in which the transport block identifier is signaled using two masks to scramble the CRC of the DCI payload bits in addition to the user id mask, this mechanism is undesirable since it reduces the number of user ids available for scheduling.
Accordingly, there is a need for an improved system and methodology for signal processing and control signaling in a MIMO system which overcomes the problems in the art, such as outlined above. Further limitations and disadvantages of conventional processes and technologies will become apparent to one of skill in the art after reviewing the remainder of the present application with reference to the drawings and detailed description which follow.